There has been known a technique in which a mat made of an insulating material is arranged between a carrier for a catalyst which is electrically energized to generate heat and a case in which the catalyst carrier is received (for example, see a first patent document). According to this mat, when the catalyst carrier is electrically energized, it is possible to suppress an electric current from flowing into the case. Incidentally, immediately after starting of an internal combustion engine, etc., a lot of particulate matter (hereinafter referred to as PM) is discharged from combustion chambers. Such PM conducts electricity, so if a lot of PM adheres to the mat, there will be a fear that electricity may flow from the catalyst carrier into the case through the PM.